This invention relates generally to a communications device and in particular to direct digitally tunable microwave oscillators and filters that enable a low-cost high-speed digital transceiver in the millimeter wave frequency range.
Telecommunication deregulation and the growth of the Internet are causing a large demand for broadband communications network access to homes and offices at increasing bit rates. For Internet access, a Digital Subscriber Loop (DSL) service and similar techniques are becoming popular. However, the speed of the DSL service is limited by the fact that the data travels over long telephone twisted copper wire pairs. Cable modems are also becoming prevalent methods of Internet and telephony access. However, the return channel for a cable modem is quite limited in speed by the nature of cable distribution technology. Fiber optics offers higher access speeds than the other systems and services, but fiber does not currently reach most homes and offices and installation of new fiber is very expensive.
To overcome these limitations with current systems, service providers and users are considering using digital wireless communications as a possible alternative access system. Various radio frequency bands are in fact currently allocated to applications that include broadband wireless access, however, most of the available bandwidth is in the millimeter wave range, ranging roughly from 15 to 66 GHz. Among these bands, the USA Local Multipoint Distributed Services (LMDS) bands in the 28 to 30 GHz range are a good example of a licensed band intended for telecom access applications. Since LMDS is a licensed band, the license owner has the exclusive rights to use the band for such services, thus ensuring interference-free operation.
LMDS services may use point to multipoint communication networks. The LMDS service provider typically maintains base station antennas on tall structures to maintain line of sight to a large number of user-buildings in a sector. Each subscribed user gets a small transceiver installed outdoors. While the base station cost is divided among many users, the subscriber transceiver serves only one user or at the most few users in a shared building. Thus, the cost of the transceiver must be kept low for the LMDS service to be economical. The option of reducing the cost of the transceiver by using a lower frequency is not always practical since it may be precluded by spectrum availability limitations so that millimeter wave transceivers are desirable, but too expensive for many applications. To make millimeter wave communications cost effective, a drastic cost reduction of the millimeter wave transceivers is required.
A typical millimeter wave transceiver includes a frequency synthesizer that generates the final millimeter wave frequency with an offset of a few GHz depending on the particular radio application. The synthesizer is required for accurately setting the exact transmit and receive frequencies of the communications. An undesirable by-product of frequency synthesis is phase noise that must be kept within acceptable performance levels for a particular radio link. Furthermore, a synthesizer may require a large tuning range, usually a few hundred MHz, to allow it to tune to alternate channels. A common way of implementing a synthesizer is to use a base phase locked loop at a lower frequency, such as around 2 GHz, and a chain of frequency multipliers. For example, a 28 GHz signal may be synthesized from a 1.75 GHz signal multiplied by 16 (i.e., 2xc3x972xc3x972xc3x972) which may be implemented using a chain of four frequency doublers.
There are several limitations to the above typical frequency synthesizer. The cost of the synthesizer is significant because the frequency doublers add complexity and cost. Furthermore, the synthesizer is very sensitive to noise. In particular, the phase locked-loop in the synthesizer has a voltage controlled oscillator (VCO). The tuning voltage of that VCO causes the final frequency of the VCO to vary by a few MHz so that one millivolt of noise in the phase locked loop control voltage may cause a frequency deviation of 500 kHz. This frequency deviation may cause temporary loss of frequency lock and large bit errors in the communications data stream which are both undesirable.
Apart from the frequency synthesizer, the millimeter-wave filters in a transceiver also increase the cost of the transceiver, especially in frequency division duplex (FDD) transceivers in which a diplexer is used. The above is also true for a time division duplex (TDD) receiver which also needs filters. Thus, it is desirable to provide a direct digitally tunable oscillator and filter that overcomes the above limitations and problems with typical oscillators and filters that may be used in millimeter wave transceiver devices and systems and it is to this end that the present invention is directed.
The tunable filters and oscillators in the microwave frequency range in accordance with the invention may include one or more tunable elements that are directly digitally controlled by a digital bus connecting a digital control circuit to each controlled element. In particular, each digital signal is filtered by a digital isolation technique so that the signal reaches the tunable elements with very low noise. The low noise digital signals are then converted to analog control voltages. The direct D/A conversion is accomplished by a special D/A converter which is manufactured as an integral part of a substrate. This D/A converter in accordance with the invention may consist of a resistor ladder or a directly digitally controlled capacitor.
The digitally controlled capacitor is tuned by electrostatic attraction in which one of the capacitor plates may bend towards a control plate or a set of control plates that causes an increase in the capacitance of the capacitor. The digital control in accordance with the invention may be achieved by partitioning the set of control plates into a plurality of sub-plates whose positions and dimensions (area) affect the overall capacitance of the capacitor such that each sub-plate represents a single weight of a weighted binary sum. In other words, a larger plate may have an area A (equal to one half the size of the other plate of the capacitor), while a smaller plate has an area of A/2, a still smaller plate has an area of A/4 and a smallest plate has an area of A/8.
To control/tune the capacitor to a desired capacitance, one or more of the plates are charged with low noise voltages (Vcc or 0) to produce the desired capacitance. For example, if only the largest plate is charged, the capacitance of the capacitor may be about xc2xd of its total value. It the first and second plates are energized (A+A/2), then the capacitor has a capacitance equal to about xc2xe of its total value. In this manner, each plate represents a binary weight (1, xc2xd. xc2xc, xe2x85x9, etc.) that may be used to control the capacitor. Any final fine-tuning of the capacitor may involve a small sub-plate driven by an analog voltage. The controlling of the capacitor in turn is used to tune a microwave circuit such as a oscillator or filter.
Various different capacitor structures are possible in accordance with the invention. In accordance with the invention, these capacitor structures are modified micro-machined parallel plate cantilever capacitors or interdigital capacitors. In addition, various combinations and orientations of the set of sub-plates relative to the other plate are possible to accomplish the desired weight distribution of the digital control word in accordance with the invention. For example, the set of sub-plates may be positioned perpendicular to the cantilever plate or parallel to the cantilever plate in cantilever capacitor in accordance with the invention.
These digitally controlled capacitors in accordance with the invention may be used to control the frequency of an oscillator or of a filter, especially in the microwave frequency range. By combining a digital tuner, a resonator and active microwave devices, a digitally controlled oscillator (DCO) is accomplished. The DCO in accordance with the invention with proper digital noise isolation may be connected to a digital circuit simplifying the implementation of a microwave frequency synthesizer by eliminating the analog control local yet allowing wide tuning range with minimum noise.
The digitally controlled devices in accordance with the invention may also be used to implement other electrically tuned devices, such as semiconductor varactor diodes. The low noise digital tuning may be accomplished by implementing a resistor ladder type of D/A converter in accordance with the invention that requires no active devices between the noise-reduced digital control word and the controlled device. The resistors may be deposited on a metal layer that acts as a shield from the circuit substrate. This can be accomplished by various well known thin film techniques. To further reduce noise, some of these resistors which are connected to the control bits may be placed outside a shielded microwave circuit containing the tunable element, and are fed to the shielded region via bypass capacitors, forming and RC low pass filter.
With direct digitally tuned microwave elements in accordance with the invention, a low cost microwave transceiver is disclosed in which some of the filters and oscillators are direct digitally tuned to reduce noise. The tuning is possible even in the transceiver front-end diplexer. The tuning in accordance with the invention may reduce the number of required diplexer filtering stages and may be used also to tune away undesired signals that are not rejected by a conventional band pass filter inside a diplexer.
In accordance with another aspect of the invention, the partitioning of the sub-plates by size (area) and position may be done with a method in accordance with the invention that includes the steps of placing a first sub-plate of a desired weight-effect where desired and adjusting the width of this plate until the desired weight-effect associated with this sub-plate is achieved and confirmed by measurement or simulation. Then a second sub-plate is placed in the presence of the first plate and its dimensions are adjusted until the desired weight-effect associated with this second sub-plate is achieved and confirmed by measurement or simulation. This process is repeated for all remaining plates to achieve the direct digitally tuned element in accordance with the invention.
Thus, in accordance with the invention, a direct digitally controlled microwave tuning element is provided comprising a microwave circuit that is being tuned to a predetermined microwave frequency and an electrically tunable element for tuning the microwave circuit to the predetermined microwave frequency. The electrically tunable element may be attached to a substrate and may further comprise a plurality of tuning signals, means for noise filtering said tuning signals to generate noise filtered tuning signals, a converter, based on the noise filtered tuning signals, that controls the frequency of the microwave circuit to a predetermined microwave frequency and an analog tuning signal that is integrated into the substrate and mounted on said noise filtering means.
In accordance with another aspect of the invention, a direct digitally controlled element for tuning to a microwave frequency is provided wherein the element comprises means for generating one or more digital signals, means for noise isolating the one or more digital signals in order to reduce the noise contained in the one or more digital signals, and means for controlling a device using the noise isolated digital binary signals, the device changing a predetermined characteristic in response to the digital signals so that the device tunes itself to a microwave frequency based on the digital signals.
In accordance with yet another aspect of the invention, a direct digitally controlled capacitor for tuning a circuit to a microwave frequency is provided wherein the capacitor comprises a control plate that deflects in response to a second plate being charged and a plurality of tuning signals attached to the sub-plates of the capacitor for controlling the capacitor. The second plate further comprises one or more sub-plates electrically isolated at DC or low frequencies from each other and from said moving plate, the sub-plates controlling the deflection of the moving plate in order to change the microwave frequency response of the capacitor. In accordance with yet another aspect of the invention, a digital microwave transceiver is provided comprising means for receiving a signal to be transmitted using a microwave frequency, and means for modulating the signal onto a predetermined microwave frequency, the modulator comprising one or more direct digitally tuned circuits for precisely controlling the predetermined microwave frequency of the modulator.
In accordance with yet another aspect of the invention, a method for determining the sub-plates areas of one or more sub-plates in a tuning device is provided, comprising positioning a first sub-plate of a largest weight-effect in a predetermined location, adjusting the dimensions of said first sub-plate until the desired weight-effect associated with said sub-plate is achieved, positioning a second sub-plate of a smaller weight-effect adjacent the first sub-plate, and adjusting the dimensions of the second sub-plate until the desired weight-effect associated with said second sub-plate is achieved.
In accordance with other aspects of the invention, a direct digitally controlled oscillator is provided wherein the oscillator comprises a tunable oscillator circuit, and a direct digitally tuned circuit connected to the oscillator circuit for controlling the frequency of the oscillator circuit. A direct digitally tunable filter is also provided wherein the filter comprises at least one resonator element, and at least one direct digitally tuned circuit electrically coupled to said resonator to control the frequency of the resonator.